A strobe device for providing a sufficient light quantity to a subject is mounted in a built-in state or separate state in a recent image pickup device such as a digital still camera.
The mounted strobe device includes a driver circuit and a flashtube having at least a trigger electrode, and instantaneously causes the flashtube to emit light in response to the voltage applied from the driver circuit to the trigger electrode.
The configuration and operation of a typical strobe device are described hereinafter with reference to FIG. 4. FIG. 4 is a diagram showing the configuration of the driver circuit for driving the flashtube of the strobe device.
As shown in FIG. 4, the driver circuit includes storage element 3 such as a capacitor, boost converter circuit 5, control section 6, trigger circuit 7, and switch circuit 8. They are in parallel with battery power supply 4. Light is emitted by applying voltage from trigger circuit 7 to trigger electrode 2 that is formed of a transparent conductive film or the like and disposed on the outer peripheral surface of flashtube 100. Battery power supply 4 supplies electric power to storage element 3. Boost converter circuit 5 increases the voltage of battery power supply 4, and control section 6 controls the whole strobe device. Trigger circuit 7 applies voltage to trigger electrode 2 with a timing of light emission of flashtube 100. Switch circuit 8 controls the emission time of flashtube 100 in response to the light emission quantity required for photographing.
In the strobe device having this configuration, when trigger circuit 7 of the driver circuit instantaneously applies voltage to trigger electrode 2 of flashtube 100, electric power stored in storage element 3 is consumed and flashtube 100 flashes and emits light. A flash of light emitted from flashtube 100 is reflected and collected directly or via a light beam control member such as a reflector, thereby instantaneously illuminating a subject.
Since a transparent conductive film as trigger electrode 2 is applied to the outer periphery of flashtube 100, however, a part of the flash of emitted light is absorbed by the transparent conductive film. Therefore, there is a problem where the radiation efficiency of the flash of light emitted to a subject decreases.
A strobe device is disclosed where, in order to prevent the radiation efficiency to a subject from decreasing, the luminous efficiency of the flashtube including a transparent conductive film as the trigger electrode is increased (for example, Patent Literature 1).
The strobe device disclosed in Patent Literature 1 is described hereinafter with reference to FIG. 5A and FIG. 5B. FIG. 5A is a front view of a conventional strobe device. FIG. 5B is a side view of the conventional strobe device.
As shown in FIG. 5A and FIG. 5B, flashtube 100 of the conventional strobe device includes an applied transparent conductive film as trigger electrode 2 only on the side of the outer peripheral surface of flashtube 100 where a subject (arrow direction in FIG. 5B) is not directly illuminated. Flashtube 100 includes, on the side where the subject is directly illuminated, non-coated section 9 to which the transparent conductive film is not applied. Flashtube 100 is mounted in the reflector having a cross section of a substantially parabolic shape in the following state:                trigger electrode 2 is disposed on the bottom side opposite to the opening side of the reflector, and the non-coated section 9 side is exposed on the opening side of the reflector.        
Thus, a member such as a transparent conductive film as trigger electrode 2 that absorbs emitted light is not disposed on the subject side of flashtube 100. Therefore, the transmission amount of the light to the subject side increases, and the luminous efficiency and radiation efficiency of flashtube 100 can be increased.
In flashtube 100 having the above-mentioned configuration, an ionized region generated in response to a trigger signal applied to trigger electrode 2 is shifted from the shaft center of flashtube 100 to the trigger electrode 2 side. The flash of light emitted from flashtube 100 is reflected while shifting to the bottom side of the reflector. Therefore, the range of the light distribution emitted to the subject becomes narrow.
When flashtube 100 is caused to emit light at a small quantity, the application time of voltage is shortened or the voltage is decreased. The position of the ionized region generated in flashtube 100 therefore fluctuates unstably. As a result, the brightness on the optical axis on the irradiated surface of the subject depends on each light emission.
A specific verification result of the light distribution characteristic is described hereinafter with reference to FIG. 6.
FIG. 6 is a diagram showing the light distribution characteristic of the conventional strobe device. FIG. 6 shows the light distribution characteristic when trigger electrode 2 is disposed on the bottom side of the reflector of flashtube 100. The horizontal axis of FIG. 6 shows angle θ (refer to FIG. 5B). The solid line of FIG. 6 shows the light distribution characteristic when the flashtube emits light at a maximum quantity, and the broken line shows the light distribution characteristic when the flashtube emits light at a small quantity, 1/256 of the maximum quantity.
According to FIG. 6, the light distribution characteristic by the light emission at the small quantity is narrower than that by the light emission at the maximum quantity.
Next, dispersion of the light brightness is described hereinafter with reference to FIG. 7. FIG. 7 is a diagram showing the dispersion of the exposure values with respect to the average value of the light emitted from the conventional strobe device. FIG. 7 shows the case where trigger electrode 2 is disposed on the bottom side of the reflector of flashtube 100 and light is emitted over 500 times at a small quantity, 1/256 of the maximum quantity. In FIG. 7, brightness at a position 1 m separate from flashtube 100 is plotted as the difference from the average value. The horizontal axis shows the number of light emissions.
At this time, the dispersion of the exposure values (EV) of FIG. 7 is calculated as “3σ=0.19EV” when the standard deviation is denoted as σ. As a result, it can be judged that light is emitted dispersedly when flashtube 100 is caused to emit light at a small quantity in the strobe device where trigger electrode 2 is disposed on the bottom side of reflector 10 of flashtube 100.
In other words, in flashtube 100 where trigger electrode 2 is disposed on the bottom side of the reflector, there is a problem where the range of the light distribution becomes narrow and the brightness does not become uniform.